WASHINGTON, D.C. — The United States stood firm Wednesday in its fight with Islamic State militants who beheaded a U.S. journalist in Iraq, pledging to continue attacking the group despite its threats to kill another American hostage. The U.S. military continued its airstrikes against the group as President Barack Obama denounced the group as a "cancer" threatening the entire region.

"We will be vigilant and we will be relentless," Obama said.

Calling for a global response to the group that now controls territory in both Iraq and Syria, Obama condemned the group's execution of journalist James Foley, whose death he said had left the nation heartbroken. In forceful remarks, Obama accused the Islamic State of torturing, raping and murdering thousands of people in "cowardly acts of violence."

"ISIL speaks for no religion," Obama said, using an alternative name for the Islamic State. "Their victims are overwhelmingly Muslim, and no faith teaches people to massacre innocents. No just god would stand for what they did yesterday and what they do every single day."

Obama's remarks affirmed that the U.S. would not change its military posture in Iraq in response to Foley's killing. Since the video was released Tuesday, the U.S. military has pressed ahead by conducting nearly a dozen airstrikes on Islamic State targets in Iraq.

The president said he'd told Foley's family in a phone call Wednesday that the United States joins them in honoring all that Foley did, praising the journalist for his work telling the story of the crisis in Syria, where Foley was captured in 2012. "Jim Foley's life stands in stark contrast to his killers," Obama said. He spoke from Martha's Vineyard in Massachusetts, where the president is on vacation.

Foley, 40, from Rochester, New Hampshire, went missing in northern Syria in November 2012 while freelancing for Agence France-Presse and the Boston-based media company GlobalPost. The car he was riding in was stopped by four militants in a contested battle zone that both Sunni rebel fighters and government forces were trying to control. He had not been heard from since.

The beheading marks the first time the Islamic State has killed an American citizen since the Syrian conflict broke out in March 2011, upping the stakes in an increasingly chaotic and multilayered war. The killing is likely to complicate U.S. involvement in Iraq and the Obama administration's efforts to contain the group as it expands in both Iraq and Syria.

The group is the heir apparent of the militancy known as al-Qaida in Iraq, which beheaded many of its victims, including American businessman Nicholas Berg in 2004.

The video released on websites Tuesday appears to show the increasing sophistication of the Islamic State group's media unit and begins with scenes of Obama explaining his decision to order airstrikes.

Page 2 of 2 - It then cuts to Foley, kneeling in the desert, next to a black-clad militant with a knife to his throat. After the captive speaks, the militant is shown apparently beginning to cut at his neck; the video fades to black before the beheading is completed. The next shot shows the captive lying dead. The video appears to have been shot in an arid area; there is no vegetation to be seen and the horizon is in the distance where the sand meets the gray-blue sky.

At the end of the video, a militant shows a second man, who was identified as another American journalist, Steven Sotloff, and warns that he could be the next captive killed. Sotloff was kidnapped near the Syrian-Turkish border in August 2013; he had freelanced for Time, the National Interest and MediaLine.